The present invention relates to a device for controlling the amount of oxidizer passing into the combustion chamber of a gas turbine engine.
It is known to provide gas turbine engines with devices for controlling the amount of oxidizer passing into the gas turbine combustion chamber. Typically, such devices include a plurality of controllable diaphragms arranged in an array around the front end of the combustion chamber and controllable so as to regulate the flow of oxidizer through the devices.
The amount of oxidizer, typically air, passing into the primary combustion zone of a combustion chamber varies widely according to the operating mode and feed conditions of the gas turbine engine. The variations are generally not proportional and cause large deviations in the richness of the air/fuel mixture between the low power and full power operating modes. During low power operation, the air/fuel mixture is lean, while at full power, the mixture is rich.
Oxidizer flow, pressure, temperature and mixture richness are comparatively low during low power operating modes. Consequently, the reaction rates within the combustion chamber are also slow. It is desirable, therefore, when in the low power mode, to restrict the oxidizer flow into the combustion chamber to enrich the air/fuel mixture in the primary combustion zone and to introduce the oxidizer into the combustion chamber at large angles in both the axial and tangential directions in order to achieve a widely distributed air/fuel mixture in order to enhance the dwell time of the gases within the combustion chamber and improve flame stability.
In the full power operating mode, the oxidizer flow, pressure, temperatures and richness of the air/fuel mixture are very high, resulting in high reaction rates. Accordingly, it is desirable to increase oxidizer flow in the primary combustion zone to reduce the richness of the air/fuel mixture and to thereby minimize the production of NO.sub.x and smoke. The air/fuel mixture is introduced into the combustion chamber at small angles in both the axial an d tangential directions thereby reducing recirculation and dwell times and rapidly terminating the reactions following combustion to minimize the combustion of NO.sub.x.
In order to accomplish these parameters, it is necessary to modulate the oxidizer flow into the combustion chamber in order to restrict the changes in richness to the primary combustion zone. In a known turbine design, each of the variable diaphragms comprises a set of vanes forming oxidizer intake ducts between them, which ducts pass through a periphery of the diaphragm structure. The diaphragm structure includes a member enclosing a periphery of the set of vanes and having circumferentially spaced apart apertures whereby relative rotation between these elements will bring the apertures into alignment with the ducts, to open the diaphragm and allow the maximum amount of oxidizer, or to move the apertures out of alignment with the ducts, thereby restricting the oxidizer input. It is also known to arrange the diaphragms in adjacent pairs and to utilize a common actuator to regulate both of the diaphragms of the pair. Typical diaphragm control devices are illustrated in French Patents 2,661,714 and 2,676,529.
In French Patent 2,676,529, the control device utilizes adjacent diaphragm assemblies in which the structure having the plurality of vanes extending therefrom and which defines the oxidizer ducts are stationarily attached to a front end portion of the combustion chamber, while the actuator is attached to the movable structure having the apertures and extending around the peripheries of the vane structure. The vane structures are identical in configuration, as are the moveable elements which rotate with respect to each other in opposite directions.
Although this structure has been generally successful, the aerodynamic profiles of the oxidizer ducts of the two diaphragm assemblies are not identical, except when the oxidizer ducts are fully closed or fully open. The lack of identity is caused by the oxidizer ducts of one of the diaphragms being situated on one side of the vane surfaces, whereas in the adjacent diaphragm, the oxidizer passages are situated on an opposite side of the vane surface. Accordingly, the tangential and axial angles of the air flow passing through the two diaphragms will not be identical to each other for a given air flow setting, thereby resulting in a non-homogeneous air/fuel mixture.